Make particles face outwards of object center?
Unfortunately you are not able to do that simply with particle system, but there is a workround:


Go to Modifier stack, clock Convert button on Particle System panel to convert these particle trees to real duplicates. Then disable the renderability and visibility button, or delete the particle system. (You may want to then deselect the earth, then group all selected trees for easy control in the following steps)
Select one of the trees, add a Track To constraint, set the "earth" as target (or use an empty object that shares the same origin location).
Use Copy Constraints To Selected Objects operation to copy the constraints to all trees. (You can find it in Objects > Constraints menu, or simply search it by hitting specebar).